Can not getSigningKey with "keyFlags(Certify)" to sign other's key

[snowyu]

The current getSigningKey method only returns the key that has keyFlags set to signData.

It is recommended to add the keyFlags option to the config parameter.

[twiss]

Hey 👋 Certifications are usually signed by the primary key. Key certifications signed by subkeys are poorly supported by most OpenPGP implementations. So, I don't think we need a keyFlags parameter for getSigningKey, rather we should change this to always use the primary key for certifications.